Question

which phrase best describes active transport?

a process that requires proteins and cellular energy input to move materials across a membrane up
a concentration gradient

a process where water molecules move across a membrane to an area of lower water concentration

a process where small molecules move across a membrane to an area of lower concentration

a process that moves materials across a membrane with the help of proteins but no additional
energy from the cell

Explanation:

Active transport is defined as the movement of substances across a cell membrane against their concentration gradient (from low to high concentration), which requires energy (e.g., ATP) and often carrier proteins. The first option matches this definition. The second option describes osmosis, the third describes simple diffusion, and the fourth describes facilitated diffusion (passive transport).

Answer:

a. a process that requires proteins and cellular energy input to move materials across a membrane up a concentration gradient